Austenite stainless steels represented by SUS304 are stainless steels having excellent corrosion resistance and workability and are most commonly used in a wide area including kitchen appliances, home electric appliances, electronic equipment, and the like. However, since the austenite stainless steels contain a large amount of Ni which is expensive due to its scarcity, there will be a problem associated with the propagation and economic efficiency of such austenite stainless steels in the future.
Meanwhile, extreme reduction of carbon and nitrogen contents in steels has become possible with recent improvement of refining technologies, and as a result, ferrite stainless steels of which the corrosion resistance and the workability are enhanced by adding a stabilizing element such as Ti or Nb are used for wide applications. A primary factor responsible for such broad applicability is that the ferrite stainless steels are superior to austenite stainless steels containing a large amount of Ni in terms of economic efficiency. However, the ferrite stainless steels are remarkably inferior to the austenite stainless steel in terms of workability, particularly elongation and uniform elongation of steel materials.
For that reason, there has recently been focus on austenite-ferrite stainless steels which lie midway between the austenite stainless steel and the ferrite stainless steel. Conventionally, the austenite-ferrite stainless steels represented by SUS329J4L still have problems in terms of propagation and economic efficiency because the austenite-ferrite stainless steels contain Ni in an amount of more than 5% and further contains Mo in an amount of several %, and Mo is scarcer and more expensive than Ni.
As an approach to cope with this problem, there has been disclosed an austenite-ferrite stainless steel wherein Mo is contained as an optional addition element and the Ni content is limited to more than 0.1% and less than 1% (Patent Document 1) or is limited to 0.5% or more and 1.7% or less (Patent Document 2). The steels disclosed in examples of these Patent Documents 1 and 2 contains N in an amount of more than 0.1%, and the Mn content is set to be in a range of more than 3.7%, for the purpose of achieving a reduction of the Ni content.
Patent Document 3 and Patent Document 4 disclose austenite-ferrite stainless steels wherein a (C+N) content or a component balance in the austenite phase is adjusted by substantially limiting the Ni content to 3% or less, for the purpose of improving total elongation and deep drawability.
Further, in this connection, examples of Patent Document 5 disclose ferrite stainless steels having excellent ductility, wherein an N content is set to less than 0.06%, a ferrite phase serves as a parent phase and a retained austenite phase is contained in an amount of less than 20%.
Patent Document 6 and Patent Document 7 disclose improvements of crevice corrosion resistance and inter-granular corrosion resistance in the austenite-ferrite stainless steel similar to that of Patent Document 3 and Patent Document 4. With regard to the steels disclosed in working examples of Patent Document 6, the Mn content is limited to less than 2%, and N is contained in an amount of more than 0.3% in the case where Ni is added in an amount of more than 0.5%. With regard to the steels disclosed in examples of Patent Document 7, the Mn content is set to be in a range of more than 2% to less than 4%, and the N content is set to be in a range of less than 0.15% in the case where the Ni content is less than 0.6%.
Conventionally, there has been pointed out in Non-Patent Document 1 that duplex steels represented by SUS329J4L which is an austenite-ferrite stainless steel taking a position between the austenite stainless steel and the ferrite stainless steel, undergoes the occurrence of furrow-like roughness along the rolling direction when subjected to a tensile processing. Here, the phenomenon is called ridging. The occurrence of ridging is closely connected with the texture of a ferrite phase, as is the case with the ferrite stainless steels. Non-Patent Document 2 and Non-Patent Document 3 address study and research on the texture of SUS329J4L.
There has been reported in these documents that the ferrite phase retains a rolling texture even after annealing of a hot rolled steel or a repetition of cold rolling and annealing, and as a result, it is difficult to obtain a re-crystallized texture. In this connection, the term “rolling texture” means strong orientation to the {001} orientation and {112} orientation, and ferrite stainless steels are easily susceptible to the occurrence of ridging if the orientation to such crystal orientations is strong. Therefore, it is considered that the occurrence of ridging in the duplex steels is also due to a strong orientation toward the rolling texture and an insufficient recrystallization of the ferrite phase, similar to the ferrite stainless steels.
Regarding the above-mentioned Patent Documents 1 to 7, there is no technology suggesting the occurrence of ridging and the texture as pointed out above. Specifically, the austenite-ferrite stainless steels disclosed in Patent Documents 3 to 7 have good formability; however, the occurrence of ridging due to processing and countermeasures thereagainst are not clearly investigated.    [Patent Document 1] Japanese Unexamined Patent Application, Publication No. H11-071643    [Patent Document 2] Specification of WO/02/27056    [Patent Document 3] Japanese Unexamined Patent Application, Publication No. 2006-169622    [Patent Document 4] Japanese Unexamined Patent Application, Publication No. 2006-183129    [Patent Document 5] Japanese Unexamined Patent Application, Publication No. H10-219407    [Patent Document 6] Japanese Unexamined Patent Application, Publication No. 2006-200035    [Patent Document 7] Japanese Unexamined Patent Application, Publication No. 2006-233308    [Non-Patent Document 1] Nippon Stainless Technical Report, vol. 21 (1986), p 12    [Non-Patent Document 2] Materials and Processes 8 (1995), p 708    [Non-Patent Document 3] Materials and Processes 17 (2004), p 408